LM358 as a differential amplifer gain not coming out right

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amad1

Joined May 25, 2019
50
Hi everybody.
I am trying to amplify some input volts(very low millivolts) to be fed to another circuit.But first i need just amplification with minimum components.

I am using LM358 amplifier but output is not as acccording to gain?

Circuit is attached .can anyone one help me out please
 

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TeeKay6

Joined Apr 20, 2019
439
Hi everybody.
I am trying to amplify some input volts(very low millivolts) to be fed to another circuit.But first i need just amplification with minimum components.

I am using LM358 amplifier but output is not as acccording to gain?

Circuit is attached .can anyone one help me out please
Clean up your schematic before we work on it. As posted there are errors, such as: D2 anode is directly connected to D2 cathode; The cathodes of D3 & D4 are connected but then go nowhere. In fact, D1-D4 do nothing but hang onto the output of U1A. Removing unneeded bends and loops makes a schematic much easier to read and to analyze. Thanks!
 

crutschow

Joined Mar 14, 2008
23,486
With a single supply voltage to the op amp, it can't amplify the negative part of the AC input.
You need to either add a negative supply for the op amp, or add a positive offset to the AC input so it never goes below ground.

Note that your circuit configuration is not a differential amp.
 

Thread Starter

amad1

Joined May 25, 2019
50
With a single supply voltage to the op amp, it can't amplify the negative part of the AC input.
You need to either add a negative supply for the op amp, or add a positive offset to the AC input so it never goes below ground.

Note that your circuit configuration is not a differential amp.
My objective is to amplify 100millivolts ac to a suffiicnet value to be compared/fed to a comparator (to drive led or relay dc),Therefore in first part i am trying to use opamp as simple amplifier as well as if it is posible to use single supply (5vdc).can you recommend circuit please?
 

TeeKay6

Joined Apr 20, 2019
439
My objective is to amplify 100millivolts ac to a suffiicnet value to be compared/fed to a comparator (to drive led or relay dc),Therefore in first part i am trying to use opamp as simple amplifier as well as if it is posible to use single supply (5vdc).can you recommend circuit please?
@amad1
It is clear that you wish to amplify a small AC voltage with a single-supply op-amp circuit. What is not yet clear is what you are trying to do with the amplified signal. Are you trying to detect a specific positive or negative peak value? ...or an average value? ...or something else? In general, a comparator provides an instantaneous indication of its input values. For example, if the signal is above the comparator threshold for 1us and then returns to a lesser value, the comparator output will be true for only that 1us period. While an LED can light for only 1us; your eyes won't see it. No relay contacts can close and open in 1us (exception, a solid-state relay). Thus you will likely need a circuit that "remembers" the event for either a specific time (e.g. from the 10us instant until 10seconds later) or until some other reset event (e.g. a pushbutton "reset" switch). So, think about what you need and then try to describe it clearly.
 

Thread Starter

amad1

Joined May 25, 2019
50
Can you please explain again, what you are trying to do?

Where did the 50 Hz signal come from?
Sir basically a series circuit consisting of Current limiting resistor(R1:110Kohms) ,neon bulb(that drops 55Volt ac across it) and a load(point a b ) having ohmic value of 55ohm.I want to measure voltage across point a b , amplify them and compare them through a comparator to drive some led or relay. If a short point a b then led should ON and if i put load(55 ohm) between point a b ,led or relay should be OFF.
But firstly i am not able to get.amplification result according to gain either on oscilloscope or ac meter in multisim?
 

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Thread Starter

amad1

Joined May 25, 2019
50
@amad1
It is clear that you wish to amplify a small AC voltage with a single-supply op-amp circuit. What is not yet clear is what you are trying to do with the amplified signal. Are you trying to detect a specific positive or negative peak value? ...or an average value? ...or something else? In general, a comparator provides an instantaneous indication of its input values. For example, if the signal is above the comparator threshold for 1us and then returns to a lesser value, the comparator output will be true for only that 1us period. While an LED can light for only 1us; your eyes won't see it. No relay contacts can close and open in 1us (exception, a solid-state relay). Thus you will likely need a circuit that "remembers" the event for either a specific time (e.g. from the 10us instant until 10seconds later) or until some other reset event (e.g. a pushbutton "reset" switch). So, think about what you need and then try to describe it clearly.
Sir basically a series circuit consisting of Current limiting resistor(R1:110Kohms) ,neon bulb(that drops 55Volt ac across it) and a load(point a b ) having ohmic value of 55ohm.I want to measure voltage across point a b , amplify them and compare them through a comparator to drive some led or relay. If a short point a b then led should ON and if i put load(55 ohm) between point a b ,led or relay should be OFF.
But firstly i am not able to get.amplification result according to gain either on oscilloscope or ac meter in multisim?

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TeeKay6

Joined Apr 20, 2019
439
Many apologies sir

Please find enclosed this one
@amad1
Your revised schematic is clearer, but there are still basic errors. A connection to ground is needed from the negative terminal of V2 and from U1A:4. U1A:2 must connect not to ground, but to the junction of R1 and R2. After those changes, others will be required. Initially, as @crutchow noted, your AC input signal has both positive (>0V) and negative (<0V) components. Your present circuit will amplify only signals that are more positive than the voltage on U1A:4 (ground). (In fact, the inputs of U1A/LM358 will allow up to 0.3V negative even with U1A:4 at ground. However, U1A cannot provide an output that is less than about 0.1V more positive than U1A:4/ground.) LM358 datasheets usually offer some example circuits of simple amplifiers. Otherwise search AAC or the Web for "LM358 circuits" or "op-amp amplifier circuits"...or some other AAC viewer may post links that could help you. The first task is to handle the input signal going negative, probably by "AC coupling" V1 to U1A via a capacitor. Try to find examples of such circuits; there are many circuit collections available online. Yes, I could give you a complete design but you would learn little from that...to use for your next project. Still, if you have specific questions, AAC viewers will help.
 

Thread Starter

amad1

Joined May 25, 2019
50
@amad1
Your revised schematic is clearer, but there are still basic errors. A connection to ground is needed from the negative terminal of V2 and from U1A:4. U1A:2 must connect not to ground, but to the junction of R1 and R2. After those changes, others will be required. Initially, as @crutchow noted, your AC input signal has both positive (>0V) and negative (<0V) components. Your present circuit will amplify only signals that are more positive than the voltage on U1A:4 (ground). (In fact, the inputs of U1A/LM358 will allow up to 0.3V negative even with U1A:4 at ground. However, U1A cannot provide an output that is less than about 0.1V more positive than U1A:4/ground.) LM358 datasheets usually offer some example circuits of simple amplifiers. Otherwise search AAC or the Web for "LM358 circuits" or "op-amp amplifier circuits"...or some other AAC viewer may post links that could help you. The first task is to handle the input signal going negative, probably by "AC coupling" V1 to U1A via a capacitor. Try to find examples of such circuits; there are many circuit collections available online. Yes, I could give you a complete design but you would learn little from that...to use for your next project. Still, if you have specific questions, AAC viewers will help.
Sir many thanks for your valuable feedback and comments...I am struck on this for last month and i have tried many circuits but in vain..that why i am putting here to get out of it..
If i use dual supply then i could cope with ac signal easily?
If you could assist me in this it would help me a lot for my next step..
 

BR-549

Joined Sep 22, 2013
4,940
You have explained how.....you want to do something. But what are you doing?

What is switching the 50 ohm load in and out of circuit?

What are you doing and why are you doing it?

There might be a much safer, easier, quicker, or better way.

If we knew what you are doing.
 

TeeKay6

Joined Apr 20, 2019
439
Sir many thanks for your valuable feedback and comments...I am struck on this for last month and i have tried many circuits but in vain..that why i am putting here to get out of it..
If i use dual supply then i could cope with ac signal easily?
If you could assist me in this it would help me a lot for my next step..
@amad1
First, correct your amplifier schematic using comments made above. Post your updated schematic. Even without correcting your circuit for the AC input signal, you should get a viewable output. If we confirm that your schematic is okay but you cannot see anything in the MultiSim oscilloscope, then you may need to seek help in a MultiSim forum. Once you get the circuit working and MultiSim displaying the output signal, then we can work on resolving the AC voltage input issue, and the "memory" issue.

Please note that working with large AC voltages is dangerous, especially so if the AC is from a power line. Be especially careful with grounding; often what "ought" to be ground is not. In your description you speak of shorting and unshorting a 55Ω resistor; please do not attempt to do that with AC power applied!
 

TeeKay6

Joined Apr 20, 2019
439
@amad1
First, correct your amplifier schematic using comments made above. Post your updated schematic. Even without correcting your circuit for the AC input signal, you should get a viewable output. If we confirm that your schematic is okay but you cannot see anything in the MultiSim oscilloscope, then you may need to seek help in a MultiSim forum. Once you get the circuit working and MultiSim displaying the output signal, then we can work on resolving the AC voltage input issue, and the "memory" issue.

Please note that working with large AC voltages is dangerous, especially so if the AC is from a power line. Be especially careful with grounding; often what "ought" to be ground is not. In your description you speak of shorting and unshorting a 55Ω resistor; please do not attempt to do that with AC power applied!
@amad1
If your only requirement is to light an LED when the 55Ω resistor is unshorted, there are indeed simpler schemes not involving op-amps & comparators. Is this truly the only thing you must do, or is using an op-amp a requirement for a solution acceptable to you? Essentially, I (and others) am still asking what exactly are you trying to do? Is this part of a larger project? Describe.
 

Thread Starter

amad1

Joined May 25, 2019
50
You have explained how.....you want to do something. But what are you doing?

What is switching the 50 ohm load in and out of circuit?

What are you doing and why are you doing it?

There might be a much safer, easier, quicker, or better way.

If we knew what you are doing.
Sir thankyou verymuch for feedback.
1) 50 OHM load is switched in manually.
2) I am trying to disntinguish(by an led or relay ) between point ab shorted to each other or above load connected.
Regards
 

Thread Starter

amad1

Joined May 25, 2019
50
hi amid,
Your circuit in post #6 is all wrong.:(
What power supplies do you have available for the project.?
E
Sit thanks a lot for your response
Currently I am having 5 V dc supply(simply adapter) ,if it is not possible through single supply then i have to use dual but guidance needed to do so..
Regards
 
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